Various flue pipe heat exchange arrangements and designs are known to extract heat from the hot flue gases leaving the combustion chamber and convected through the flue pipe extending in the water tank to transfer the heat from the flue gases to the water within the tank which surrounds the flue pipe. Baffles are commonly installed in the flue pipe to increase the heat transfer by slowing the speed of the flue gases while increasing turbulence thereof for longer contact of the hot gases with the flue pipe wall and the baffle plates mounted therein. Various baffle designs have been developed to achieve this improved heat transfer. However, condensation develops in the flue pipe and this is undesirable. An ideal design is to prevent or substantially minimize the formation of condensation in the vertical flue pipe section directly above the combustion chamber while extracting maximum heat from the flue gas to heat the water in the tank. Condensation is known to cause corrosion of the steel and the baffle plates.
Several know designs have been patented to increase the efficiency of the water heater by the extraction of heat from the flue pipe as it exits the water heater tank. The extracted heat may be used to heat water circulated from the bottom of the water tank to the top of the tank. However, those known system designs have their limitations and usually the flue gases are exhausted to atmosphere at higher than desirable temperatures. Ideally, the flue gases should be exhausted at about 100 degrees F., to provide for an efficient heat transfer system. It is also known to extract heat from the hot flue gases for other purposes than for heating the water in the tank of the water heater. For example, the extracted heat may be used for local heating of the air or heating another liquid in a heat exchanger.
More recently, different approaches have been suggested wherein the flue pipe is configured to have two or more sections extending through the water in the water tank. These are commonly referred to as multi-pass condensing type flue designs. However, these designs have resulted in other problems such as occupying extra space in the water tank and therefore reducing the tank capacity for water or increasing the size of the tank to accommodate the extra flue length and therefore adding to increase costs. They have also resulted in condensation evacuation problems. The extra costs of the flue pipe and its installation cost is not recoverable after the life of the water heater.